DESCRIPTION (Investigator's Abstract): The importance of studying synaptic function at the molecular level is most obvious for understanding mental and neurological diseases where psychopharmacological therapeutics, modern moleculargenetics and biochemically oriented neuropathology suggest an underlying synaptic malady. Long-term presynaptic facilitation, which is a form of synaptic plasticity underlying behavioral sensitization in the marine mollusk, Aplysia, and an elementary form of learning, can be produced by the action ofthe cAMP-dependent protein kinase (PKA).A decrease of 20-50 percent in the regulatory (R) subunits of PKA occurs in sensory nerve cells when stimulated to produce long-term facilitation; this molecular change is sustainedonly if new protein is made. No change in catalytic (C) subunits occurs. A decreased R/C ratio would be expected to produce a kinase more sensitiveto subsaturating levels of cAMP: the change in R/C sets the baseline extent of protein phosphorylation in the neuron at a higher level. How is this fine control of cAMP-dependent phosphorylation achieved? There is no changein the levels of PKA subunit mRNA in sensory cells that have been facilitated, rather R subunit protein is selectively degraded. The first aim of the present application is to characterize the mechanisms causing turnover of R biochemically. Preliminary studies suggest that R subunits are degraded by ubiquitin-mediated proteolysis. It is likely that this process is enhanced degradation. The second aim is to identify the factor(s) that enhance degradation of R subunits in sensory neurons that have received stimuli which produce long-term facilitation. The first set of experiments proposed is designed to show definitively that R subunits are ubiquitinated and then to characterize the proteolytic mechanism involved. The second set of experiments is designed to determine what factors target the degradation of the R subunits. Specifically targeted proteolysis may be a general mechanism for generating the persistent protein phosphorylation common to other forms of long- termsynaptic plasticity. A similar mechanism is thought to degrade selectively other proteins that regulate cell division, cancer, cell growth and determination of body shape. If correct, this idea is a further indication that similar molecular mechanisms are used both in learning and in development.